Session Initiation Protocol (SIP) is an application layer signaling and control protocol for establishing and managing delivery of Internet Protocol (IP)-based telephony services in a packet-based network. SIP provides user authentication, redirect and registration services, and can be used to support a variety of telephony services such as audio or video conferencing, text messaging, interactive gaming, and call forwarding. The SIP protocol is described in Request for Comment (RFC) 3261, published in 2002, and prepared by the Internet Engineering Task Force (IETF).
SIP provides several functions for the setup, modification, and termination of sessions. The functions include registration, feature negotiation, and call management. The registration function enables a client to discover its current location within a network. A SIP registrar processes registration requests for the purpose of updating a location server with location information for a client specified in the request. The registrar updates the location server by writing a binding between a universal resource identifier (URI) and an IP address associated with the client.
The registration function allows the SIP network to locate clients. For example, when an client launches a messaging application, the client sends a register message to the registrar. The register message includes the URI for the client, the IP address of the client, and a registration timer. The registrar stores the binding between the URI and the IP address for the duration of the registration timer. Typically, a registration is valid for a predetermined period of time, e.g., 3600 seconds. In order to guarantee that the client is reachable, the client re-registers prior to the expiration of the registration timer.